Several market dynamics have come together recently to create an environment for significant change in the computer and telecommunication industries. Explosive growth of the Internet has led to changes in the way regional telephone companies (telcos) view their networks. The Telecommunications Act of 1996 spawned an environment where entrepreneurs can capitalize on niche openings, and large corporations (e.g. Public Utilities) find themselves in position to offer new revenue generating services. Technical advances in personal computers and network router technology have created an environment where next generation multimedia applications are ready to skyrocket. Recent corporate mergers have brought the cable industry to the forefront of the telecommunications market while other mergers and new market entrants have shown that the long haul backbone is moving to packet based technology.
These changes taken independently result in interesting evolutionary steps that can be implemented within the framework of existing product platforms and processes. Taken as a whole, however, they beckon a revolution—a new class of telecommunications products and a new network paradigm.
Until the late 1970's analog techniques were used in the Public Switched Telephone Network (PSTN). The Customer Premise Equipment (CPE), a residential phone or private branch exchange, were analog and the circuits connecting the CPE to a telco central office were analog. Analog techniques were used to switch telephone calls within the central office as well as between central offices.
Digital switches and ISDN introduced techniques for converting analog speech to digital bit streams. The conversion was standardized. Digital techniques supplanted analog and were used to switch telephone calls within a central office as well as between central offices.
At the sending end, the analog to digital conversion standard requires sampling the analog signal every 125 microseconds and coding the sample into an eight-bit number that represents the analog amplitude. The sampling and coding follow recommendation G.711. The coded eight bit numbers are placed in 64 KBPS channels. For analog circuits coding occurs in the interface card at the central office and for ISDN lines coding occurs at the CPE. This function is often referred to as a Code/Decode or CODEC function.
The G.711 recommendation specifies 64 KBPS per conversation, which for standard human conversations is overkill. The average human conversation requires as little as 4 KBPS to transmit all audible communications using other CODEC technologies. Several other CODECs have been developed including G.722, G.723.1, G.728, G.729, or the GSM coding standard G.729a, often used in IP telephony. Some of these recommendations were developed to help deal with situations where not as much bandwidth was available to carry the voice conversation which is the case in analog and digital cellular environments.
A PSTN digital telco central office receives 64 KBPS calls from TDM circuits (or converts analog circuits to 64 KBPS) and switches each 64 KBPS channel toward a terminating user. If the local central office serves the terminating user, the channel is sent to the local line circuit. If a distant central office serves the terminating user, digital trunks carry the 64 KBPS information transparently to the central office that services the user.
At the receiving end, the digital to analog conversion standard requires that the 64 KBPS channel information be used to reconstruct an analog signal following the G.711 recommendation. The CODEC used (e.g., G.711) at the sending end dictates what CODEC to use at the receiving end. The same standard must be used to decode the 64 KBPS (or other rate) signal as was used to encode it.
During call establishment, bandwidth is reserved from end to end. All of the network components in the call path reserve bandwidth based on which CODEC is in use, and the network components are all synchronized. Network components include the analog to digital converters (CODECs), digital switches, and digital trunks.
Voice packet switching (e.g., IP telephony) is quite different. Historically, packet switching did not reserve bandwidth between end users nor were packets switched based upon packet priority. Standards work in this area is ongoing and these capabilities do exist for some network types.
At the sending end, one method for analog to packet conversion is described by H.245. H.245 permits a call to be handled using G.711, G.722, G.723.1, G.728, G.729, or GSM coding standard. When multiple coding standards are supported by the endpoints, H.323 provides a protocol to resolve which coding standard will be used for any specific call.
A local area network receives packets from connected endpoints or gateways. These packets are routed toward a terminating endpoint or gateway. If the terminating address is not on the same local area network, the packet may be routed via a number of routers, switches, connecting trunks, etc.
At the receiving end, packet to analog conversion is required. The receiving end must use the same conversion standard that was used at the sending end. During call establishment, bandwidth may or may not be reserved from end to end. Network components (hubs, routers, switches, etc.) may or may not implement priority-based packet switching. Thus, there is no guarantee of the end to end packet delay. Voice packets may arrive at the receiving end with different inter-packet arrival times and occasionally out of sequence.
Although each subscriber in the PSTN may appear to have dedicated bandwidth, there are concentration points within the network. Sophisticated engineering algorithms have been developed to help network planners determine where in the network to locate concentration points and what the concentration factors should be. The Erlang is a unit of measurement that describes the utilization of TDM circuits within the PSTN. One Erlang is equivalent to one TDM circuit in use 100% of the time.
Prior to the popular past time of dialing into your local Internet Service Provider (ISP), PSTN engineering rules assumed that the average residential subscriber received or placed 2 phone calls per hour and each call lasted an average of 3 minutes. This yielded an average Erlangs per subscriber line of 0.1. Of the 0.1 Erlangs, 10% were assumed to be originated or terminated outside of the subscriber's central office. In practical terms, this means that (in general) for every 10 Erlangs of subscriber line use, 1 Erlang of trunking capacity to other central offices is required.
With the proliferation of Internet service providers (ISPs), this relationship changed drastically. During the evening hours when Internet use is heavy, a much higher percentage of subscriber traffic leaves the subscribers central office. This change caused telcos to look for alternate means of providing data access to subscribers.
One means at the Local Exchange Carrier's (LEC) disposal is to use a technology known generically as Digital Subscriber Loop or (xDSL). This technology allows analog voice signals to coexist with digital data signals. Subscribers can be directly connected to a data network over their existing twisted pair without interrupting their ability to place phone calls on that same pair. The LEC could then groom the data traffic off the network at the line access point and eliminate the high demand for inter office trunks. In addition, this opened a new revenue stream for LECs by giving them incentive to become ISPs. The deployment of xDSL has been slow, however, and other high speed Internet access methods are on the horizon.
The 1996 Telecommunications Act ushered (or more accurately “is ushering”) in a new era of competition in the local service market. A new breed of communications company has been created known as Competitive Local Exchange Carriers or CLECs. Many of the initial CLECs have built their business around offering local service exclusively to the lucrative enterprise business market. The residential market has continued to be monopolized by the LEC.
Regulations bodies have been slow to force LECs to open their local loops at reasonable prices and it's impractical for most CLECs to consider building out a residential network.
Other access means for the “last mile” to the subscriber residence have been slow to develop. Wireless companies are constrained by available RF bandwidth. Cable operators have just recently been pushing data modems into their cable offerings and had considered the “lifeline” telephony service too large a hurdle to overcome.
Initially, cable modems were created to help bring high-speed data access to subscribers that were stuck with 28.8 KBPS modem access. Some ventures have had moderate success, as cable data services have been slow to deploy. Problems with the agreement on standards as well as with old cable plants that did not support 2-way communication made progress slow.
Today, coaxial cable passes 98% of the homes in the United States. 60% of U.S. households subscribe to cable services. In addition, Coaxial Cable Modem standards (known as DOCSIS) currently can provide 2 to 3 times the bandwidth of ADSL technology and are being deployed more quickly.
As competition in the local markets begins to heat up, and with LECs wanting to get into the long distance business, LECs and Inter Exchange Carriers have been consolidating and merging to remain strong. Most Network Access Provider mergers seem to be driven by competition. Traditional telecommunications vendors have been merging with traditional data network vendors to create “Unified Networks” vendors that can deliver both circuit and packet switched networks. Competition and convergence are the two forces driving change in the telecommunications and data communications industries. These two forces often tend to be cumulative in their impact on both traditional Network Access Providers and traditional network infrastructure vendors.
The foregoing illustrates a brief evolution of circuit switched voice and packet switched voice and data services as provided to residential and business subscribers. It is evident that most of the intelligence of these systems is centralized.